NOTES.md

DomminusCarnufex rs-kernel

This one adds a syscall to the Linux kernel and rebuilds it. The important steps:

• add a Makefile rule to build an rlib (an ar archive) using rustc, specifying #![crate_type="rlib"] in the source
• extract the .o file from the rlib, and rename it to match the expected name
• add the .o file to obj-y

They also edit the syscall interrupt handler (in asm) to dispatch to their syscalls.

No reference to lang items. It uses stable #[no_std] without #[no_core] - I'd expect the resulting object file just happens not to reference libcore, but more complicated objects could.

No custom compiler flags etc. other than #[no_std].

saschagrunert/kmod

A custom Makefile drives the build, by copying a Makefile.in that looks like a normal Linux module Makefile to a build directory, and then invoking the usual make -C /lib/modules/$(uname -r)/build M=$PWD (see Documentation/kbuild/modules.txt ).

The custom Makefile sets RUSTCFLAGS to -O -C code-model=kernel -C relocation-model=static.

There's also a module.c file containing the usual MODULE_LICENSE etc. macros, plus prototypes for init_module and cleanup_module.

Makefile.in does the following:

• build the module with cargo rustc, specifiyng crate_type = ['staticlib'] in Cargo.toml
• set foo-objs := module.o libfoo.a (note that order is significant: as with shared libraries, static libraries need to come after things that depend on them)
• set obj-m := foo.o

(The foo-objs syntax is for "composite objects," which is documented in Documentation/kbuild/makefiles.txt section 3.3, except for the bit where -objs is synonymous to -y; see scripts/Makefile.lib for that. It works by combining the listed objects with ld -r aka --relocatable to generate foo.o.)

I couldn't get the out-of-tree build to work right with my 3.16 (Debian oldstable) kernel; it builds fine if you get rid of the extra Makefile (rename Makefile.in to Makefile) and get rid of the src variable there, making it build in-tree.

kmod includes two other source files besides lib.rs and module.c, which aren't very interesting. lang_items.rs has ~empty implementations of all three lang-items, and print.rs has a straightforward FFI binding to printk, plus a macro called println! that doesn't actually implement the format-string logic. These should be wired up to Linux's unwinding / OOPSing mechanisms and to core::fmt respectively.

I can replicate the Makefile parts of this by themselves. The following Makefile successfully links a kernel module out of one.rs and two.c:

obj-m := three.o
three-objs := two.o libone.a
lib%.a: %.rs
	rustc -o $@ -O $<

(Since the point of this Makefile is just to make linking work, I'm being lazy and not specifying #![no_std] in the Rust source; libstd gets copied into libone.a but the linker eliminates it as dead code when building three.c. That lets me avoid thinking about lang-items.)

tsgates/rust.ko

This one is much older (2013, last updated 2016) but much more complete. It brings up a couple of important points:

• You need a custom target file to set things like disable_redzone and no use of floating point. (Their custom target file also sets code-model to kernel and relocation-model to static, as with the previous one.)
  • Apparently you can just use "+soft-float" in target features these days, See this comment on rust-lang/rfcs#1364.
• The kernel internal ABI has no guarantees at all, and you want to write against the kernel C API, using bindgen.
  • One limitation is that bindgen doesn't support C macros.

pmj/rustykext

MacOS. Also uses -C soft-float and -C no-redzone=y (and -C no-stack-check, which is no longer necessary).